1. Field of the Invention
The present invention relates to a steering device for a vehicle in which transmission ratio between a steering angle of a steering member and a turning angle of turning wheels can be varied.
2. Related Art
As the steering device for a vehicle as described above, a structure including a first shaft connected to a steering wheel, a second shaft connected to vehicle wheels, and a steering device with variable transmission ratio which interconnects the first shaft and the second shaft has been known (Reference should be made to Japanese Patent Publication No. JP-A-2005-53446, for example). In JP-A-2005-53446, the steering device with the variable transmission ratio is provided with a wavy motion gear mechanism.
As the steering device with the variable transmission ratio, a structure employing a planet gear mechanism, in place of the above described structure including the wavy motion gear mechanism, is taken into consideration. Specifically, it is considered to employ the planet gear mechanism which includes, for example, a first sun gear fixed to the first shaft, a second sun gear fixed to the second shaft, a plurality of pinions arranged around the first and second shafts, and a carrier in an annular shape for supporting the pinions.
Rotation of a steering member is transmitted to the first shaft, the first sun gear, the pinions, the second sun gear, and the second shaft, in this order, and finally transmitted to the vehicle wheels. In this case, when the carrier is actuated by applying torque to the carrier from an electric motor by way of a speed reducing gear mechanism, ratio between rotation numbers (change gear ratio) of the first sun gear and the second sun gear can be varied. In this manner, the transmission ratio is varied.
In the above described structure, when an abnormal condition such as breakdown of the electric motor has occurred, it becomes impossible to control the transmission ratio. In such a case, it is possible to fix the carrier by locking gears of the speed reducing gear mechanism, thereby to fix the change gear ratio constant and to fix the transmission ratio constant.
By the way, the inventor of this application has come to an idea, as the result of his keen researches, that the carrier is rotated using the electric motor of a so-called coaxial type. Specifically, by arranging a rotor and stator of the electric motor coaxially with the steering shaft, output rotation of the rotor is transmitted to the carrier without interposing the speed reducing mechanism.
However, in case where the electric motor of the coaxial type is used, the speed reducing mechanism is not employed, and the transmission ratio cannot be fixed by locking the speed reducing gears.